1. Field of the Invention
The present invention relates to head gimbal assemblies in a hard disk drive.
2. Background Information
Modern disk drives include a servo controller driving a voice coil actuator to position a read-write head near a track on a rotating disk surface. The read-write head communicates with the servo controller, providing feedback, which is used in controlling the read-write head's positioning near the track. The read-write head is embedded in a slider, which floats on a thin air bearing a very short distance above the rotating disk surface.
The trend in the hard disk drive industry is to increase the areal density of the rotating disk surfaces. This is usually achieved by decreasing the flying height of the read-write head above the rotating disk surface. Currently, read-write heads fly at 10 nano-meters (nm) from the rotating disk surfaces.
There are problems with flying the read-write heads and sliders so near the rotating disk surfaces. Contact between the read-write heads and the disk surface they access, needs to be minimized, to insure the reliability of the hard disk system as a whole. For example, the read-write heads may contact the disk surface, which tends to reduce the reliability of the data stored there, and may possibly damage the read-write head as well.
A voice coil actuator typically includes a voice coil, which swings at least one actuator arm in response to the servo controller. Each actuator arm includes at least one head gimbal assembly typically containing a read-write head embedded in a slider. The head gimbal assembly couples through a load beam to the actuator arm in the voice coil actuator.
A hard disk drive may have one or more disks. Each of the disks may have up to two disk surfaces in use. Each disk surface in use has an associated slider, with the necessary actuator arm. Hard disk drives typically have only one voice coil actuator.
Today, the bandwidth of the servo controller feedback loop, or servo bandwidth, is typically in the range of 1.1 K Hz.
Extending servo bandwidth increases the sensitivity of the servo controller to drive the voice coil actuator to finer track positioning. Additionally, it decreases the time for the voice coil actuator to change track positions.
However, extending servo bandwidth is difficult, and has not significantly improved in years. As areal densities increase, the need to improve track positioning increases.
One answer to this need involves integrating a micro-actuator into each head gimbal assembly. These micro-actuators are devices typically built of piezoelectric composite materials, often involving lead, zirconium, and tungsten. The piezoelectric effect generates a mechanical action through the application of electric power. The piezoelectric effect of the micro-actuator, acting through a lever between the slider and the actuator arm, moves the read-write head over the tracks of a rotating disk surface.
The micro-actuator is typically controlled by the servo-controller through one or two wires. Electrically stimulating the micro-actuator through the wires triggers mechanical motion due to the piezoelectric effect. The micro-actuator adds fine positioning capabilities to the voice coil actuator, which effectively extends the servo bandwidth. The single wire approach to controlling one micro-actuator provides a DC (direct current) voltage to one of the two leads of the piezoelectric element. The other lead is tied to a shared ground. The two wire approach drives both leads of the piezoelectric element of the micro-actuator.
There are two approaches to integrating the micro-actuator into a head gimbal assembly. Embedding the micro-actuator between the slider and the load beam, creates a co-located micro-actuator. Embedding the micro-actuator into the load beam, creates a non co-located micro-actuator. The non co-located micro-actuators tend to consume more power, requiring higher driving voltages than the co-located micro-actuators.
A problem arises when integrating micro-actuators into hard disk drives with multiple disk surfaces. Each of the micro-actuators requires its leads to be controlled by the servo-controller. These leads are coupled to wires, which must traverse the main flex circuit to get to the bridge flex circuit. The bridge flex circuit provides electrical coupling to the leads of the micro-actuator.
The main flex circuit constrains many components of the voice coil actuator. If the shape or area of the main flex circuit is enlarged, changes are required to many of the components of the actuator arm assembly and possibly the entire voice coil actuator. Changing many or most of the components of an actuator arm assembly, leads to increases in development expenses, retesting and recalibrating the production processes for reliability, and inherently increases the cost of production.
The existing shape and surface area of the main flex circuit has been extensively optimized for pre-existing requirements. There is no room in the main flex circuit to run separate control wires to each micro-actuator for multiple disk surfaces. This has limited the use of micro-actuators to hard disk drives with only one active disk surface.
What is needed is a way to minimize the time that the read-write heads fly close to the rotating disk surfaces they access. What is further needed, is a way to integrate micro-actuators into multiple disk surface disk drives using the existing surface area and shape of the main flex circuit.